From the Terminal Norte bus station:

Fastest/Easiest: A taxi costs about $10.000 pesos (about $5.00 USD). Tell the driver to go to Parque Lleras and then to take Calle 7 up the hill two blocks to the hostel. Please keep in mind that all taxi drivers in Medellin are required to always use the Taxi meter.

Cheaper: By Metro/Bus combination. In the Caribe Metro Station by the Terminal Norte, buy a "Poblado Integrado" ticket: they will give you one ticket for the Metro and another for a Metro bus. Take the Metro south to the "El Poblado" Metro Station. Once there, catch a bus that says "Metro" - using your other ticket - to take you up the hill. Get off at Dominoes Pizza on Carrera 36. Walk down Carrera 36 two blocks to Calle 7 and you will see Casa Kiwi Hostel on the left.

From the Terminal Sur bus station AND the Olaya Herrera local airport:

Fastest/Easiest: A taxi costs about $6.000 pesos (about $3.00 USD). Tell the driver to go to Parque Lleras and then to take Calle 7 up the hill two blocks to the hostel. Please keep in mind that all taxi drivers in Medellin are required to always use the Taxi meter.

Cheaper: I do not recomend this. It is pretty far and, like anywhere, it might be dangerous at night. If you really enjoy walking with a heavy backpack, it is possible to walk (about 45 minutes) from the Terminal Sur to the hostel. Follow Calle 10 all the way over the river/highway and then up the hill to Carrera 36 and Dominoes Pizza. Take a right on Carrera 36 and walk two blocks. Casa Kiwi Hostel will be on your left at the end of the street.

From the Rionegro/Jose Maria Cordova international airport:

Fastest/Easiest: A taxi costs about $55.000 pesos (about $27.00 USD). This is one of the few instances where there is a set taxi price, and the driver will not use the Taxi meter. Ask the driver how much the fare will be before getting into the taxi to make sure he doesn´t overcharge you. Tell the driver to go to Parque Lleras in El Poblado and then to take Calle 7 up the hill two blocks to the hostel.

Cheaper: You can also take a "collectivo" bus from the airport to the center of Medellin for about $10.000 pesos (about $5.00 USD), and then a taxi from the center to the hostel that should cost about $7.000 (about $3.50 USD). Tell the driver to go to Parque Lleras and then take Calle 7 up the hill two blocks to the hostel.